Ultrastructural studies of regenerating spines of the sea urchin Strongylocentrotus purpuratus I. Cell types without spherules

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Abstract
The fine structure of regenerating tips of spines of the sea urchin Strongylocentrotus purpuratus was investigated. Each conical tip consisted of an inner dermis, which deposits and contains the calcite skeleton, and an external layer of epidermis. Although cell types termed spherulecytes containing large, intracellular membrane bound spherules were also present in spine tissues, only epidermal and dermal cell types lacking such spherules are described in this paper.The epidermis was composed largely of free cells representing several functional types. Over the apical portion of the tip these cells occurred in groups, while proximally they were distributed within longitudinal grooves present along the periphery of the spine from the base to the tip. The terminal portions of apical processes extending from some of the epidermal cells formed a thin, contiguous outer layer consisting of small individual islands of cytoplasm bearing microvilli. Adjacent islands were connected around the periphery by a junctional complex extending roughly 200 Å in depth in which the opposing plasma membranes were separated by a narrow gap about 145 Å in width bridged by amorphous material. Other epidermal cells were closely associated with the basal lamina, which was 900 Å in thickness and delineated the dermoepidermal junction; some of these cells appeared to synthesize the lamina, while others may be sensory nerve cells.The dermis at the spine tip also consisted of several functional types of free cells; the most interesting of these was the calcoblast, which deposits the skeleton. Calcoblasts extended a thin, cytoplasmic skeletal sheath which surrounded the tips and adjacent proximal portions of each of the longitudinally oriented microspines comprising the regenerating skeleton, and distally, formed a conical extracellular channel ahead of the mineralizing tip. The intimate relationship between calcoblasts and the growing mineral surface strongly suggests that these cells directly control both the kinetics of mineral deposition and morphogenesis of the skeleton. Other cell types in the dermis were precalcoblasts and phagocytes. Precalcoblasts may function as fibroblasts and are possible precursors of calcoblasts. Closely associated with the basal lamina at the dermoepidermal junction were extracellular unbanded anchoring fibrils 150 Å to 200 Å in diameter. Scattered proximally among dermal cells were other extracellular fibrils, presumably collagenous, about 300 Å in diameter with a banding periodicity of 210 Å.